This invention relates generally to computed tomography (CT) imaging and more particularly, to dose verification in an imaging system.
In at least one known CT system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a "view". A "scan" of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector.
In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts that attenuation measurements from a scan into integers called "CT numbers" or "Hounsfield units", which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time, a "helical" scan may be performed. To perform a "helical" scan, the patient is moved while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a one fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed.
Certain safety tests are typically required to be completed prior to delivering, after delivery, and when certain components are replaced. One such test is verification of the x-ray dose received by an object or patient. In at least one known CT system, where the same reference object is scanned, the detector intercepts all of the direct x-ray exiting from the source collimator, and the detector x-ray energy sensitivity and gain are reasonably flat, the imaging system can be considered a dosimeter. As a result, image noise can be used for dose verification. If, however, the three described conditions are not met, for example, where a portion of the direct x-ray beam is not received by the detector, x-ray dosage cannot be verified by measuring only image noise.
Accordingly, it would be desirable to provide a system for facilitating dosage verification where the direct x-ray beam extends beyond the edges of the detector. It would also be desirable to provide such a system without increasing the cost of the system.